Dozens of people have been injured in unrest that broke out as Enrique Pena Nieto took office as Mexico's new president.

Authorities said at least 76 people were injured and around 100 detained as police battled protesters who hurled gasoline bombs and smashed shop fronts.

Demonstrators had rallied on inauguration day on December 1 to oppose the return to power of Pena Nieto’s Institutional Revolutionary Party (PRI), which ruled Mexico for more than 70 years until 2000.

The party is notorious among many Mexicans for ruling with an authoritarian hand and allowing corruption to flourish.

In his inaugural address, Pena Nieto, 46, vowed to end years of drug-related violence that has left tens of thousands of people dead.

He presented a 13-point plan that includes measures to boost economic growth, counter inequality, and improve the standard of living for millions of Mexico’s impoverished.

Pena Nieto, whose term is for six years, succeeds Felipe Calderon, who launched the war against the drug cartels that smuggle cocaine, marijuana, and crystal methamphetamine, mainly to the United States.

More than 60,000 people are estimated to have been killed in the drug war in the past six years in Latin America’s second-biggest economy after Brazil.

Many Mexican cities and towns see frequent gunfights, kidnappings, and massacres connected to the war.

Addressing concerns about the return of the PRI, Pena Nieto pledged that his government will be open and responsive to all sectors of society and would firmly defend human rights.

He spoke of taking a different approach to resolving Mexico’s problems, saying the country of more than 113 million people needs a new “social pact.”

"We need to change the paradigm, understand that there won't be security while there is no justice," he said. "In the Mexico of the future, there is justice and inclusion that will form the basis of a social pact."

Pena Nieto pledged increased investment to improve infrastructure and expand Mexico’s participation in the global economy. The country has one of the top 15 biggest economies in the world, but much of the population continues to live below the poverty line.

He promised to create an interagency national crime prevention program to rebuild the country's social fabric.

"Mexico needs to live in peace. I am convinced that authorities not only battle crime, it is essential that the state becomes an integral force to reconstruct the social fabric," he said.

In Moscow, the Kremlin confirmed that President Vladimir Putin had congratulated Pena Neito, saying that "Mexico is an important partner of Russia in Latin America." Russian Security Council head Nikolai Patrushev headed Russia's delegation to the inauguration.

Patrushev told journalists he had discussed several bilateral agreements in talks with Pena Neito, including a possible visa-free-travel pact and a draft memorandum on security cooperation.

The second-place finisher in the July election, leftist Andres Manuel Lopez Obrador, on December 1 repeated allegations that Pena Nieto and the PRI had won the vote through electoral fraud.

A court has rejected claims that the vote was won illegally. But Lopez Obrador has refused to concede defeat.